The present invention concerns a system for resampling one sampled data signal to generate another sampled data signal where the two sampled data signals have different sample rates, and in particular, to such a system which resamples signals having respective sampling rates that may be represented as a ratio of two large relatively prime integers.
Sample rate conversion systems are well known for converting information bearing signals from one format to another code. Such systems are commonly used for data telemetry, audio signal processing, and video signals standards conversion.
An exemplary video signal standards conversion system is described in U.S. Pat. No. 5,057,911 entitled, SYSTEM AND METHOD FOR CONVERSION OF DIGITAL VIDEO SIGNALS, which is hereby incorporated by reference for its teachings on digital resampling techniques.
The system described in the above-referenced patent converts video signals having a nominal sample rate of 13.5 MHz into a digital signal having a sample rate of 14.31818 MHz, (hereinafter 14.3 MHz) which is compatible with the National Television Standards Committee (NTSC) Standard. The ratio of these two sample frequencies is exactly equal to the ratio of 33 to 35.
Accordingly, the standards conversion system described in the referenced patent generates 35 output samples for every 33 input samples that it receives. This is accomplished using 35 interpolation filters which generate interpolated samples at 35 respective positions between any two of the input samples. The filter, X.sub.i+1, which is selected to generate the i+1.sup.th output sample is defined by equation (1). EQU X.sub.i+1 =(X.sub.i 33) MOD 35 (1)
This technique of sample rate conversion works well when the input and output sample rates can be represented as a ratio of two relatively small numbers. When, however, the ratio of these rates cannot be represented to a high degree of precision by a manageable ratio, this conversion process may be inaccurate or unduly expensive to implement.
For example, consider a resampling circuit which converts from the CCIR (601) standard having a nominal sample rate of 13.5 MHz to the Phase Alternate Line (PAL) standard which has a sample rate of 17.734475 (hereinafter 17.7) MHz. The smallest exact ratio between these two sampling frequencies is 540,000 over 709,379. Thus, if the scheme described above were used to resample the CCIR (601) signal into a PAL digital signal, 709,379 digital filters would be required. At the current state of the art, it is impractical to implement a video standards conversion system having 709,379 filters.
An alternative sampling scheme may be to use a ratio which approximates the desired ratio and to periodically reset the resampling system, for example, once per horizontal line in order to prevent errors from propagating. This scheme, however, is relatively complex since the starting sample interpolation filter changes from line to line. In addition, this scheme may result in an excessively distorted image causing the individual lines of the image to be either shorter or longer than they would be if the proper sequence of interpolation filters were used.